1. Field of the Invention
This invention relates to a method for preparing pure, thermally stable and high surface area ceria. The ceria can be a catalyst, a catalyst support or incorporated into a catalytic washcoat.
2. Description of the Related Art
Automotive three-way catalytic converters are employed to convert pollutants produced by internal combustion engines into harmless emissions. The catalyst must be highly active for converting CO, NO and unburned hydrocarbons into nitrogen, carbon dioxide and water over a wide range of exhaust compositions. As a result, a noble metal catalyst that is usually platinum, rhodium and/or palladium must be employed. To utilize the noble metal efficiently, it is dispersed on a high surface area support such as γ-alumina typically having a surface area greater than 150 m2/g. The high surface area support also provides a surface for the deposition of poisons like lead, zinc and phosphorus that are usually present in the exhaust gas. Since the catalyst may reach temperatures as high as 1000° C., the support must be thermally stable and resistant to sintering.
The composition of the engine exhaust gas oscillates between net oxidizing (lean) conditions for air/fuel ratios higher than the stoichiometric one and net reducing (rich) conditions for air/fuel ratios lower than the stoichiometric one. However, the catalyst operates most efficiently on a stoichiometric exhaust gas. Ceria is commonly added to three-way catalysts to promote the activity of the noble metals under these transient conditions; and similarly, Ce4+ can be reduced to Ce3+ under rich conditions. In this manner, ceria tends to dampen down oscillations in the exhaust gas stoichiometry.
Ceria may be incorporated into a three-way catalyst washcoat either by impregnation of the high surface area alumina support with a cerium salt or by physically mixing bulk cerium oxide with the other catalyst components. The addition of bulk ceria to the catalyst may be preferable since interactions between impregnated ceria and the alumina support decrease the oxygen storage capacity of the ceria. However, at the extreme temperatures (in excess of 1000° C.) often encountered in catalytic converters, ceria rapidly sinters to surface area of less than 5 m2/g. Therefore, the incorporation of a thermally stable, high surface area ceria would prove beneficial to three-way catalyst performance.
U.S. Pat. No. 3,830,758 discloses the importance of using a high surface area support for palladium and platinum oxidation catalysts. Ceria with a purity of 98% was prepared by precipitation of cerium nitrate with ammonium hydroxide at a pH of 9. While the ceria had a high surface area of 218 m2/g after calcination at 427° C. in air for 4 hours, the surface area dropped to merely 4 m2/g following 4-hour calcination at 982° C.
U.S. Pat. No. 4,661,330 discloses a method for preparing high surface area and high purity ceria. Ammonium ceric nitrate is refluxed for 24 hours with ammonium sulfate to obtain a hydrous ceria powder. Following calcination at 538° C. in air, this material maintains a surface area of 150 m2/g. However, the stability of this material at higher calcination temperatures was not investigated. In EP 0444470A1, pure ceria was prepared with the same method as in U.S. Pat. No. 4,661,330 for comparison. After calcination at 980° C. for 4 hours, the surface area of the prepared ceria remained only 5 m2/g.
U.S. Pat. No. 4,859,432 teaches a method for preparing a morphologically improved ceria having high surface area. Ceria is produced by reacting a cerium salt with a strong base in the presence of carbonate ions followed by calcination. After calcination at 600° C., ceria with a surface area of 117 m2/g was obtained. Higher calcination temperatures were not investigated. In EP 0444470A1, pure ceria was prepared with the same method as in U.S. Pat. No. 4,859,432 for comparison. After calcination at 980° C. for 4 hours, the surface area of the prepared ceria remained only 1 m2/g.
In EP 0444470A1, pure ceria was prepared by cerium nitrate decomposition at 538° C. in air for 1 h. After calcination at 980° C. for 4 h, the ceria remained a surface area of only 1.4 m2/g.
Accordingly, the above products in the prior art all have relatively low surface areas after calcination at 980° C. for hours, so that the catalytic effect is poor.